1. Technical Field
The present invention relates to a mode-locked laser device, an ultrashort pulse light source device, a broad bandwidth light source device, a non-linear optical microscopy device, a recording device and an optical coherence tomography device, and particularly relates to a mode-locked laser device that outputs ultrashort pulse light and an ultrashort pulse light source device, broad bandwidth light source device, non-linear optical microscopy device, recording device and optical coherence tomography device.
2. Related Art
Ultrashort pulse light with pulse widths of picoseconds or femtoseconds is used in applications that utilize non-linear effects, such as secondary photon absorption induced by the very large peak powers, second harmonic generation (SHG), coherent anti-Stokes Raman scattering (CARS) and the like.
A technique for generating ultrashort pulse light is the “mode-locking” method, in which a solid-state laser medium disposed in a resonator is excited by a semiconductor laser or suchlike and the phases of numerous oscillating longitudinal modes are synchronized. A laser device that generates pulsed laser light with this method is referred to as a mode-locked laser device.
As mode-locked laser devices that are currently commercially available, TSUNAMI, manufactured by SPECTRA-PHYSICS, CHAMELEON, manufactured by COHERENT, and so forth are widely used. However, these mode-locked lasers are laser devices that are based on titanium-sapphire crystals and have complex resonator structures, as illustrated in FIG. 17.
Because these resonator structures are complex and green solid-state lasers are used for excitation of the titanium-sapphire crystals, numbers of components are large and the laser devices themselves are very expensive, being ten million yen or more. Moreover, in regard to output stability, feedback functions have to be added to optimize the resonator mirrors and make outputs constant in response to output variations, and these are unstable such that oscillation ceases after several weeks. Furthermore, the laser devices are large, being tabletop size.
In these mode-locked lasers, a mode-locking technique referred to as Kerr lens mode-locking is employed. However, with this technique, it is difficult for the mode-locking to self-start, and a driving mechanism for causing self-starting (corresponding to the AOM (acousto-optic modulator) in FIG. 17) must be provided. Moreover, in order to induce mode-locking, structuring for conditions in which the resonator destabilizes is necessary, and this is one of the causes of the aforementioned complexity of the resonator structure and of output destabilization.
In recent years however, mode-locked lasers that use a component known as a semiconductor saturable absorbed mirror (SESAM) have been reported. By using this component, it is possible to easily make mode-locking self-starting, and the mode-locking can be stably applied. Moreover, because this component functions as a resonator mirror, the resonator structure can be simplified, and a low-cost mode-locked laser with a compact and simple resonator structure can be realized.
FIG. 18 (see Optics Letters, vol. 29, pp. 2629-2631 (2004)) illustrates a mode-locked laser that uses a semiconductor saturable absorber mirror (SESAM hereinafter). As shown in FIG. 18, the resonator has a linear structure, the two ends of which are constituted by a concave mirror 200 and a SESAM 202. Only three components constitute the resonator—the concave mirror 200, the SESAM 202 and a solid-state laser medium 204—which is an extremely simple structure. Therefore, a lowering of costs and a reduction in size are possible. Furthermore, because the resonator length is short at a few cm, even if the resonator mirror and the like are displaced by environmental changes in temperature, humidity and the like, displacement of the resonator optical axis is suppressed, and a very high stability mode-locked laser can be realized. The resonator length of the commercially available mode-locked laser using titanium-sapphire that is illustrated in FIG. 17 is long, at around 2 m, which is another major factor in destabilization of output of the mode-locked laser, and the resonator length has a large effect on output stabilization.
Now, in order to start or maintain mode-locking in a mode-locked laser that uses a SESAM, a pulse energy Ep inside the resonator must be maintained at least a mode-locking threshold energy Ec,p expressed by equation (S1) (see J. Opt. Soc. Am. B, vol. 16, pp. 46-56 (1999)).Ec,p=Fsat,L*Fsat,S*AL*AS*ΔR  (S1)
Here, Fsat,L is the saturation fluence of the laser medium, and is expressed as hν/σ, using the Planck constant h, an oscillation light frequency ν and a stimulated emission cross section σ of the laser medium. Fsat,S represents the saturation fluence of the SESAM, AL represents an oscillating light beam cross section in the laser medium, AS represents the oscillating light beam cross section at the SESAM, and ΔR represents the modulation depth of the SESAM.
The pulse energy Ep in the resonator is expressed by the following equation, using an average output power Pout, an output mirror transmittance T and a repetition frequency of the pulse light frep.Ep=(Pout/T)/frep  (S2)
The repetition frequency frep is a number of round trips of the pulse light in the resonator in a unit of time, and is expressed by the following equation, using a resonator length L and the speed of light c.frep=c/2L  (S3)
As is clear from the above equations (S2) and (S3), the shorter the resonator length, the smaller the pulse energy Ep in the resonator. If the pulse energy Ep in the resonator falls below the mode-locking threshold energy Ec,p expressed by the preceding equation (S1), the mode-locking ceases to be in effect. Therefore, in order to shorten the resonator length given a constrained average output power Pout, it is necessary to reduce the mode-locking threshold energy as much as possible.
To reduce the mode-locking threshold energy, it is necessary to reduce the respective parameters in equation (S1). Fsat,S and ΔR are characteristics of the SESAM. Minimum values of these in SESAMs that are currently commercially available are Fsat,S=70 μJ/cm2, and ΔR=0.4%, approximately. AL and AS are at least about 2.85×10−5 cm2. If the cross section were reduced further, the resonator would become unstable, and there would be a risk of destroying the SESAM. Therefore, to obtain a mode-locked laser with a shorter resonator length, an Nd-doped laser medium with which the stimulated emission cross section σ can be made larger and Fsat,L can be made smaller is used (see, for example, Japanese National Publication No. 2002-536823).
However, with an Nd-doped laser medium, the stimulated emission cross section is large but the oscillation bandwidth is narrow. Therefore, even at the shortest, optical pulses shorter than several picoseconds cannot be generated. In recent years however, a Yb-doped laser medium that can be excited by a high-output infrared semiconductor laser has attracted interest as a laser medium for a high-output ultrashort pulse mode-locked laser. This laser medium has a wide oscillation bandwidth, and can generate optical pulses of hundreds of femtoseconds. A difference in pulse width between picoseconds and hundreds of femtoseconds is similar to values of peak power being an order of magnitude different. When considering, for example, secondary photon absorption, this order of magnitude difference means a difference of two orders of magnitude in absorption. Thus, this difference in pulse widths is significant for non-linear applications.
However, the Yb-doped semiconductor laser medium has a small stimulated emission cross section σ, and with a mode-locking threshold energy derived from the following equation (1), it is difficult to make the resonator length less than 15 cm.
                                          c            ×                          E                              c                ,                p                ,                s                                      ×            T                                2            ×                          P              out                                      ≤        L        <                              c            ×                          E                              c                ,                p                                      ×            T                                2            ×                          P              out                                                          (        1        )            
In other words, it has been difficult to realize a small-size ultrashort pulse mode-locked laser with a pulse width of hundreds of femtoseconds and a resonator length of less than 15 cm. That is, it has been difficult to make the length of a resonator in a simple structure as illustrated in FIG. 18 shorter and to realize an ultrashort pulse (hundreds of femtoseconds) mode-locked laser that is compact and low in cost.